I. Field of the Invention
The present invention relates generally to optical fiber, and more particularly to supplying coating material to a coating applicator in the fabrication of optical fiber.
II. Description of Related Art
Methods for the manufacture of optical fibers by drawing the fiber from a glass preform and coating the fiber with a liquid polymer immediately after drawing are well known. The polymer coating serves as a protective coating to prevent airborne particles from impinging upon and adhering to the surface of the drawn fiber, which would mechanically weaken the fiber or even affect its transmission properties. Also, the coating shields the fibers from surface abrasion, which could occur as a result of subsequent manufacturing processes and/or handling during installation. The coating also provides protection from corrosive environments and spaces the fibers in cable structures.
Optical fibers are coated with the liquid polymer coating material by a process which typically involves passing the fiber through an applicator having a coating die that defines a small, vertically orientated, longitudinal tapered passage. The coating material is injected under pressure into the coating applicator so that the coating material is applied uniformly about the fiber at a predetermined thickness. The coating material is cured or otherwise solidified to allow the fiber to be reeled for further processing or storage. The cure mechanism predominantly used in commercial practice with liquid polymer coatings is UV induced polymerization. It is common to apply only a single coating to the fiber or to apply two coatings either sequentially or simultaneously (typically referred to as the primary and secondary coatings).
With recent emphasis on increasing fiber draw efficiency, larger performs are being utilized so that the draw process can produce continuous fiber of greater lengths without having to stop the process to load a new perform. It is known that stopping the draw process results in reduced yield, increased down time and an increase in wasted materials (e.g., glass and coating material).
In many fiber manufacturing processes, the coating is supplied in batch, from a reservoir having a limited amount of coating material. The capacity of the these reservoirs was determined based upon the preform size at the time the drawing system was designed and built. Because of concerns for space efficiency and cost, the reservoirs were originally designed to hold only enough material to cover the fiber drawn in a single run from a single perform of the size known at that time. Because some reservoirs require a settling period of at least several hours once filled to allow any air bubbles in the coating material to rise to the surface and be released (referred to as off gassing), many draw systems incorporated two reservoirs for each coating (e.g., two reservoirs for the primary and two reservoirs for the secondary) so that the reservoirs can be switched when a new perform is loaded. Thus, when a glass perform is exhausted and the drawing process is stopped to load a new perform, the reservoir supplying the coating material is switched. The switch is typically made by selectively opening and closing the appropriate valves to and from the full reservoir the empty reservoir. For those coating applications which do not use gravity to feed the coating material to the coating die, the appropriate valves are opened and closed so that the desired amount of air pressure is applied to the full reservoir to force the coating material through a series of conduits to the coating applicator at a precise pressure. The drawing process then starts drawing fiber from the new perform which is coated by coating material supplied by the full reservoir. The empty reservoir is then filled and off gassed during a settling period while the other reservoir supplies the coating material.
With the increase in preform size, existing reservoirs are not able to provide enough coating material for a complete run of a single preform. Thus, since the reservoirs typically operate independently of one another, once the reservoir supplying the coating has been emptied, the draw process is stopped to switch to the other reservoir which has been filled and off gassed. This results in wasted materials (e.g., glass and coating material) as well as additional down time. Accordingly, it has been recognized that the fiber draw capacity is bounded by the size of the coating reservoir in use.
One proposed solution is to increase the reservoir size. While possibly a viable solution for new fiber draw towers to be built, this generally is not a practical solution for existing draw towers. The addition of larger reservoirs would require additional space, and may require modifications to the piping and other equipment located near the reservoir location. In addition, new reservoirs would have to be purchased and installed, which would be costly, especially with systems that fabricate dual coated fibers (thereby requiring four or more reservoirs per tower). Furthermore, this modification would only last until preform technology again surpasses coating capacity.
Thus, there exists a unsatisfied need in the industry for a cost effective solution for increasing the amount of coating material that can be continuously supplied during the fabrication of optical fiber.